This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Doxycycline, a therapeutic drug used to treat bacterial infections and has been shown to decrease inflammation. The purpose of this study is to assess the safety and effectiveness of doxycycline and to determine if doxyxyxline can decrease inflammation in cystic fibrosis patients. Selected CF patients will receive doxycycline study drug treatmetn for a period of 28 days. Subjects will be closely monitored through blood draws, pulmonary function tests, physical exams, and follow up phone calls throughout the 28-day period. In addtiona, patients will be followed for 2 weeks following the doxycycline administration.